The manufacture of castings for internal combustion engines poses difficult manufacturing problems. For example, the cylinder head of an internal combustion engine, whether for a spark-driven gasoline internal combustion engine or a compression-ignition diesel engine, is a complex article of manufacture with many requirements. A cylinder head generally closes the engine cylinders and contains the many fuel explosions that drive the internal combustion engine, provides separate passageways for the air intake to the cylinders and for the engine exhaust, carries the multiplicity of valves needed to control the air intake and engine exhaust, provides a separate passageway for coolant to remove heat from the cylinder head, and can provide separate passageways for fuel injectors and the means to operate fuel injectors.
The walls forming the complex passageways and cavities of a cylinder head must withstand the extreme internal pressures, temperatures and temperature variations generated by the operation of an internal combustion engine, and must be particularly strong in compression-ignition diesel engines. On the other hand, it is desirable that the internal walls of the cylinder head, particularly those walls between coolant passageways and the cylinder closures, permit the effective transfer of heat from the cylinder head.
It is also important that all castings for internal combustion engines include minimal metal to reduce their weight and cost. The countervailing requirements of reliable internal combustion engine parts make casting such parts difficult. Furthermore, these complex parts are manufactured by the thousands and assembled into vehicles that must operate reliably under a variety of conditions. Consequently, the casting of internal combustion engine parts has been the subject of the developmental efforts of engine and automobile manufacturers throughout the world for years.
Cylinder heads are most generally manufactured by casting them from iron alloys. The casting of the cylinder head portion that closes the cylinders, carries the intake and exhaust valves and fuel injectors and provides the passageways for the air intake, exhaust and coolant requires a mold carrying a plurality of core elements. To provide effective cooling of the cylinder head and effective air intake and exhaust from the cylinders of the internal combustion engine, the passageways for the air intake and exhaust are best interlaced with the coolant passageways within the cylinder head portion. The cavities for coolant, air intake and exhaust must, of course, be formed by core elements within the mold that can be removed when the casting metal solidifies.
Such core elements are formed from a mixture of core sand and a curable resin, which, when cured, retains the shape imposed on it prior to curing, and after a casting solidifies, the core sand and resin residue are removed from the casting.
As a result of recent developments, core assemblies are provided by a plurality of core elements that have interengaging surfaces to locate the plural core elements in the core assembly. For example, head core assemblies can be formed by the assembly of a one-piece coolant jacket core, a one-piece exhaust core, and a one-piece air intake core that interengage during their assembly; however, to maintain such an assembly together as a unit during post-assembly handling and casting, the core elements must be fastened together. Because of the high rate of manufacture of internal combustion engines and the stringent requirements for their reliability, such fastening methods must be both rapidly effected and reliable. In the past, adhesive and/or screws have been used to fasten core elements together to maintain the integrity of the core assembly during its handling and during pouring of the casting.
The use of an adhesive requires an adhesive that can be easily spread on the core elements, that will set within the shortest possible time, that will hold the core elements together as one piece and maintain their position during the casting process, and that may be removed from the casting after the casting metal solidifies. This method results in substantial costs and opportunities for unreliable castings because of a potentially unreliable interface between the core elements. The adhesive materials may separate or otherwise become degraded in storage. It is also necessary that workmen apply the adhesive correctly so that the adhesive reliably maintains the core elements together during casting and is not spread onto an exposed casting surface. Furthermore, this method requires time for applying the adhesive, assembling the core elements together and allowing the adhesive to set before the core elements can be used for casting, and it introduces into the mold a foreign element in the form of an adhesive that may evolve gas that may become trapped in the solidified casting and cause areas of possible failure.
Because of the difficulties of using adhesive to fasten core elements together, screws have been used to fasten the core elements of core assemblies together. Although the use of screws to fasten core elements together provides a more predictable assembly of the core elements than adhesive, the use of screws requires the installation of accurately sized pins in the mold-form for the core to provide accurately sized holes in the core to accept the screws. Such pins in the mold-form become eroded by the abrasive core sand and bent in use, resulting in holes in the core that are too small or that cannot accept screws from an automatic installation station. As a result, screws frequently fail to properly engage the core sand core elements and to provide holding engagement of the core sand elements as a result of core sand stripping during their installation.